1. Field of the Invention
This invention is directed to a method for activating carbon and thermally reactivating activated carbon and, more particularly, a technique for enhancing the thermal reactivation of activated carbon that once served in water treatment for the removal of taste and odor causing compounds (TandOs; e.g., 2-methylisobomeol and geosmin), volatile organic compounds (VOCs; e.g., benzene, xylenes, and toluene), synthetic organic chemicals (SOCs; e.g., atrazine and lindane), and naturally occurring organic matter (NOM).
2. Description of the Related Prior Art
Activated carbon both in the powdered (PAC) form (generally defined as 90% passing the 325 mesh) and granular (GAC) form (generally defined as passing the 8 mesh, but retained on the 30 mesh or passing the 12 and retained by the 40 mesh) has been used extensively during the past several decades for the removal of unwanted compounds from drinking water. Increase in activated carbon use occurred in the late 1970""s upon the U.S. EPA""s recommendation of it as being the best available technology (BAT) for controlling trihalomethanes and, later, SOCs in contaminated ground water and drinking water. However, GAC has a finite adsorption capacity, and approaches a point where it cain no longer remove the organics required to purvey aesthetically pleasing water that also meets the EPA""s stringent water quality standards.
After GAC has exhausted its finite adsorption capacity or when users deem it necessary, a common practice is to reactivate and return the activated carbon back to service. Typically, spent activated carbon is reactivated in a rotary kiln furnace, but also can be reactivated in fluidized bed or multiple hearth furnaces. Conventional thermal reactivation includes the following steps as discussed by Suzuki et al. xe2x80x9cStudy of thermal regeneration of spent activated carbons: Thermogravimetric measurement of various single component organics loaded on activated carbonsxe2x80x9d Chlem Eng Sci 1978;33(3):271-279. First, the wet carbon is dried at 105xc2x0 C. to release water. Second, the GAC is pyrolyzed in a starved gas environment between 650 and 850xc2x0 C. During pyrolysis, volatile compounds that accumulated during operation are released. This step also causes fragments of adsorbed organic compounds on the GAC surface to form a carbonaceous char. Finally, the adsorbed char is oxidized and gasified by exposing the GAC to C02, steam, or a combination of both at 650 to 900xc2x0 C. The inherent limitation of this oxidation step is that it gasifies a fraction of the carbon surface while it is gasifying the char. In other words, some of the carbon is burned during thermal reactivation.
Activated carbon""s excellent performance in removing numerous organic compounds has been proven, but it is common to hear the words xe2x80x9cactivated carbonxe2x80x9d and xe2x80x9cexpensivexe2x80x9d in the same sentence. Thermal reactivation can often represent the largest expense associated with using GAC.
Therefore, a method that can reactivate activated carbon that decreases the mass and volume loss, results in a BET surface area or iodine number (as measured by ASTM D4607) near its virgin counterpart, and that lasts longer for removing compounds compared to its virgin counterpart presents an opportunity to decrease the costs associated with thermal reactivation. In other words, if mass loss and volume loss could be decreased during thermal reactivation, then less virgin carbon make-up would be required to replace the carbon lost during reactivation. If the reactivated carbon could stay in service for longer periods of time, then reactivation frequencies would decrease, which would decrease costs because reactivation cycles would be farther apart. Finally, if the reactivated carbon""s iodine number and/or BET surface area are close to the virgin counterpart, then the carbon could experience more thermal reactivation cycles. Similarly, a method that improves the efficacy of activated carbon for removing unwanted compounds (such as those listed above) presents an opportunity to improve water treatment.
Therefore, an object of the present invention is a method for reactivating activated carbon which decreases the mass and volume loss yet results in a BET surface area or iodine number near its virgin counterpart: the reactivated carbon lasting longer for removing compounds compared to conventionally reactivated carbon and, in some instances, its virgin counterpart.
Another object of the present invention is a method for the development of an activated carbon superior in removing unwanted compounds to improve water treatment compared to those that are activated conventionally (i.e., pyrolysis followed by steam).
This object and other objects are achieved by a method for reactivating activated carbon, comprising the steps of steam treating the activated carbon followed by pyrolysis, both the steam treatment and pyrolysis being conducted at a temperature within the range of about 400xc2x0 C. to about 900xc2x0 C.
An additional aspect of the invention includes a method for reactivating activated carbon, comprising the steps of pyrolysis followed by steam treating the activated carbon, both the steam treatment and pyrolysis being conducted at a temperature within the range of about 400xc2x0 C. to about 900xc2x0 C., wherein the steam treatment comprises treating the activated carbon with steam prepared from water having a dissolved oxygen (DO) content of less than about 9 mg of oxygen per liter of water.
Another aspect of the invention includes a method for activating a carbonaceous material, comprising the steps of steam treating the carbonaceous material followed by pyrolysis.